The invention is based on a priority application EP 04 290 907.7 which is hereby incorporated by reference.
The present invention relates to the field of wireless telecommunication systems and more particularly without limitation to the usage of the transmission control protocol (TCP) over wireless links.
The transmission control protocol (TCP) originally has been designed and tuned for networks composed of wired links and stationary hosts. However, usage of TCP in a mobile wireless environment has also been considered in the prior art (cf. TCP (transmission control protocol) over wireless links, Vehicular Technology Conference, 1997 IEEE 47th Chan, A. C. F.; Tsang, D. H. K.; Gupta, S. Pages: 1326-1330 vol. 3).
In an Ethernet LAN (IEEE 802.3), the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) protocol establishes how simultaneous transmissions (packet collisions) will be handled. In a WLAN, collision detection in this manner is not possible due to the fact that a station is not able to transmit and listen at the same time. To account for this difference, 802.11 uses a slightly different protocol known as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) or the Distributed Coordination Function (DCF). CSMA/CA is based on a “listen before transmit” policy and attempts to avoid packet collisions by using explicit packet acknowledgement (ACK), which means that an ACK packet is sent by the receiving station to confirm that a packet arrived intact.
CSMA/CA works by having the station that wishes to transmit senses the air and if there is no activity detected, the station will wait an additional random period of time and if there still is no activity, will transmit the data. If the packet is received intact, the receiving station will send an ACK frame that once it is received by the original sender the transmission is complete. If the ACK command is not received in a specified random period of time, the data packet will be resent, assuming that the original packet experienced a collision.
A particular problem is created by the “near-far” effect in WLAN systems. Such systems face the so called near-far problem; the presence of a remote user in the cell in a low bit rate area e.g. at 1 megabit per second can greatly degrade the throughput of a high data rate user. The aggregate throughput of the cell is also affected. This phenomenon is due to the fact that all mobile terminals within a given cell have equal access probability to the same transmission medium. For equal packet sizes a packet transmitted at 1 megabit per seconds usually occupies the transmission medium 11 times longer than a packet transmitted at 11 megabits per second.